1. Field of the Invention
The present invention relates to a method for producing a liquid crystal display panel.
2. Description of the Related Art
In recent years, there have been demands in the market for: a liquid crystal display panel capable of displaying motion pictures for use in personal computers and multimedia applications; a liquid crystal display panel capable of performing a high definition display (e.g., SVGA or XGA) and performing a large screen display; and a large liquid crystal display panel which can be used in place of a CRT in a desk-top computer system. In order to meet such demands, it is required for a liquid crystal display panel, especially an STN (Super Twisted Nematic) color liquid crystal display panel, to have high performance characteristics such as a high contrast (e.g., 30:1 or greater), a high brightness, a high response speed (e.g., a response time of 200 ms or less), a high display quality and a low power consumption.
The quality display characteristic, among other characteristics, is difficult to improve since it is in a trade-off relationship with high contrast and high brightness. Moreover, as the response speed of a liquid crystal display is improved, the quality display characteristic has to be improved under more demanding conditions. Equalizing the cell gap of the display panel (corresponding to the distance between a pair of substrates) has been one way of improving the quality display characteristic, the cell gap issue being one of the highest priority display problems. One particular problem which is associated with the cell gap non-uniformity which must be solved is the color irregularity seen near an injection seal, where the cell gap is more likely to be non-uniform.
According to a well-known method, a liquid crystal display panel is produced by: printing a plurality of injection seals on one of a pair of substrates so as to surround a plurality of liquid crystal injection areas (corresponding to a plurality of liquid crystal cells); attaching the substrate to the other substrate and pressing the substrates together; and severing the substrates into a plurality of liquid crystal cells.
As the area of a single liquid crystal cell increases, however, the number of liquid crystal cells produced from a pair of substrates decreases, thereby also decreasing the total area of the injection seals. As a result, an excessive load may be applied on the injection seal during the pressing process. In such a case, when the injection seal is heated and cured after the pressing process, the injection seal greatly expands and changes its shape. Moreover, such an excessive load may also deform the gap material, thereby reducing the production yield and significantly increasing the cost of the product.
In order to solve such problems, Japanese Laid-open Publication No. 64-9426 describes: providing an auxiliary seal around an injection seal on one of a pair of substrates; attaching the substrate to the other substrate; scribing and breaking the substrate; and removing the auxiliary seal, thus producing at least one liquid crystal cell. This publication does not particularly pursue a preferred shape of the auxiliary seal, but only describes that the seal can take a shape such as a triangle, a rectangle, a pentagon, other polygons, a circle or an ellipse. The publication further describes that the auxiliary seal is preferably formed in such a manner that a load is applied uniformly on the injection seal when the pair of substrates are attached together. However, this publication does not describe the precision (uniformity) of the cell gap in the vicinity of the injection seal when the panel is completed. In fact, it was not possible to equalize the cell gap of the panel in the vicinity of the injection seal when the auxiliary seal, as described in this publication, was provided, for example, in the vicinity of the injection seal.
Japanese Laid-open Publication No. 8-313917 describes an injection seal in a rectangular pattern surrounding a liquid crystal injection area, wherein the rectangular pattern has a relatively small width along each of the short sides thereof and a relatively large width along each of the long sides thereof. This structure prevents the deformation of the liquid crystal injection area due to the difference in load per unit area between the short side and the long side of the rectangular pattern which occurs when the pair of substrates are pressed together. This publication further describes an auxiliary seal provided in a linear pattern around the injection seal. However, it was not possible to equalize the cell gap of the panel in the vicinity of the injection seal by following the description of this publication (i.e., by providing an injection seal in a rectangular pattern surrounding a liquid crystal injection area, with a relatively small width along each of the short sides thereof and a relatively large width along each of the long sides thereof).
It is not possible to sufficiently diffuse the stress acting upon each substrate by providing an auxiliary seal in a large pattern or by providing an auxiliary seal in a linear pattern. It should also be noted that, when attaching the substrates together, the air therebetween is discharged more easily through some positions in the substrate than through other positions thereof. Moreover, when attaching the substrates together, there occurs a difference in the amount of expansion between the auxiliary seals, thereby making the cell gap non-uniform.
Japanese Laid-open Publication No. 5-188387 discloses another method which prevents the cell gap non-uniformity or a crack in the substrate from occurring in the severing process. In accordance with this method, a thermosetting adhesive mixed with hard spacers such as glass beads is applied on one of the substrates by a screen printing method or by using a dispenser so as to form an injection seal (along the periphery of a liquid crystal panel) and an auxiliary seal for preventing the substrates from being peeled from each other (in an area outside the liquid crystal panel, which area is to be severed from the panel). Relatively soft spacers such as plastic beads are dispersed on the entire surface of the other substrate. Then, an UV-curable adhesive is applied on the area outside the liquid crystal panel which is to be severed from the panel so as to form a temporary securing section. Thereafter, the pair of substrates are attached together and irradiated with an UV radiation so as to cure the UV-curable adhesive and thus to temporarily secure the substrates together. A plurality of such pairs of substrates are stacked on top of one another and are pressed and heated so as to cure the thermosetting adhesive. Thus, a predetermined cell gap is obtained. Then, the pair of substrates is scribed and subjected to a break process so as to divide the substrates into a liquid crystal display panel and the remaining portion outside the liquid crystal display panel.
In accordance with the method disclosed in Japanese Laid-open Publication No. 5-188387, an injection seal 61 along the periphery of a liquid crystal display panel 60; an auxiliary seal 62 and a temporary securing section 63 provided in an outer area outside the liquid crystal, display panel 60; and a scribe line 64 between the liquid crystal display panel 60 and the outer area are provided in a positional relationship as illustrated in FIG. 21. One or two auxiliary seals 62 are provided in parallel with a side of the injection seal 61 with the same width as the width of the injection seal 61 of that side. For example, only one auxiliary seal 62 may be provided near the injection seal 61 or, alternatively, two auxiliary seals 62 may be provided; one near the injection seal 61 and the other near the temporary securing section 63. Each auxiliary seal 62 is formed in a linear pattern and is discontinuous wherever the scribe line 64 is present. The temporary securing section 63 is formed outside the auxiliary seal 62. The temporary securing section 63 does not overlaps with the scribe line 64 and is usually formed in the vicinity of the scribe line 64.
In accordance with the method disclosed in Japanese Laid-open Publication No. 5-188387, after the attachment process, the temporary securing section 63 has a circular shape with a diameter of about 5 mm and a thickness or height of about 10 .mu.m or greater. This thickness is considerably greater than the cell gap of the liquid crystal display panel (e.g., about 6 .mu.m). Moreover, the temporary securing section 63 is already cured in a pillar-like shape before the pressing and heating process. Therefore, despite the provision of the one or two linear auxiliary seals, the cell gap of this liquid crystal display panel rapidly increases near the temporary securing section 63.
Furthermore, it has been found that when the temporary securing section 63 is provided near the scribe line 64 and the linear auxiliary seal 62 is discontinuous in and near the area where the scribe line 64 is present, the stress distortion influence from the temporary securing section 63 is conducted to the inside of the liquid crystal display panel via sections where the linear auxiliary seal 62 is discontinuous. In such a case, a cell gap variation is caused in the vicinity of each corner of the liquid crystal display panel.
The temporary securing section 63 takes on such a thick pillar-like shape, as described above, because the temporary securing section 63, which consists of an UV-curable adhesive, is cured while the pair of substrates are attached together with a relatively weak force. This is believed to be causing the situation where the thickness of the temporary securing section 63 is not reduced to a predetermined cell gap of the liquid crystal display panel (even when the substrates are pressed and heated thereafter). It is believed that the cell gap of the liquid crystal display panel rapidly increases near the temporary securing section 63 because of the large area and the large thickness of the temporary securing section 63 and because of the great cell gap difference between the temporary securing section 63 and the injection seal 61 which makes the substrate wavy.
As described above, there is a need for a method for producing a liquid crystal display panel which provides a uniform cell gap in the vicinity of the injection seal and which reduces or eliminates the cell gap variation caused by, for example, the stress distortion due to the temporary securing section 63.